A magnetoresistive head is used as a read sensor in a highly recording density magnetic recording technology mainly for a hard disk; and constitutes a part that largely affects the performance of a magnetic recording technology. In recent years, the magnetoresistance effect of a multilayer film formed by laminating ferromagnetic metal layers with a nonmagnetic metal layer in between has become substantial, and the resulting “giant magnetoresistance effect”, is well known. In this case, electric resistance varies in accordance with the angle formed between the magnetizations of the two ferromagnetic layers interposing the nonmagnetic intermediate layer. When such a giant magnetoresistance effect is applied to a magnetoresistive element, a structure called a spin valve is advocated. A spin valve structure: takes the structure of an antiferromagnetic layer/a ferromagnetic layer/a nonmagnetic intermediate layer/a ferromagnetic layer; and makes it possible to substantially pin the magnetization of the ferromagnetic layer touching the antiferromagnetic layer through a magnetic exchange coupling field generated at the interface between the antiferromagnetic layer and the ferromagnetic layer, freely rotate the magnetization of the other ferromagnetic layer through an external field, and thereby obtain an output. Here, a ferromagnetic layer wherein the magnetization is substantially pinned by an antiferromagnetic layer is defined as a pinned layer and a ferromagnetic layer wherein the magnetization rotates through an external field is defined as a free layer. The aforementioned basic construction is also commonly adopted in a GMR that is currently used, specifically a CIP-GMR, a tunneling magnetoresistive element (a TMR), and a current perpendicular to plane GMR (a CPP-GMR).
Further, in a read sensor, a pair of magnetic shields is disposed in the manner of interposing the magnetoresistive element in order to select a magnetic field to be sensed from all the external fields. In a current perpendicular to plane magnetoresistive head such as a CPP-GMR or a TMR, a structure wherein electric current is applied to a sensor through the aforementioned paired magnetic shields is generally adopted.
In a current perpendicular to plane magnetoresistive head, it is known that gigantic noise called spin torque noise caused by spin conduction is generated in accordance with a relative angle between the magnetization of a free layer and the magnetization of a pinned layer and the direction of the flow of electric current as shown in M. Covington, M. AlHajDarwish, Y. Ding, N. J. Gokemeijer, and M. A. Seigler, Phys. Rev. B, vol. 69, pp. 184406-1-184406-8, 2004; J. G. Zhu and X. Zhu, IEEE Trans. Magn., vol. 40, pp. 182-188, 2004; and N. Smith, J. A. Katine, J. R. Childress, M. J. Carey, IEEE Trans. Magn., vol. 41, pp. 2935-2940, 2005.